The present invention relates to electric energy meters, and more particularly to a method and apparatus for compensating for timing errors in an electronic time of use or demand energy register in the energy meter.
Electric energy meters frequently include both a watthour meter and an electronic register which provides time of use or demand of electric energy which is used for billing purposes. Since the time of use information provided by such electronic registers is used in determining customer billing, it is important that such energy meters are highly accurate, and avoid errors in the billing information. Power companies desire to level out, or average, peak energy demands by their customers, and billing rates are frequently established which charge less for energy in recognized, off-peak periods, and/or which penalize a customer, such as an industrial customer, for peaks in energy demand and consumption. Accordingly, it is important that the electronic register not only accurately measure energy consumption, but that the timekeeping provided in such electronic registers be accurate.
It is common to utilize the power line frequency to provide the primary timing signal for the electronic register since power line frequency accuracy is carefully maintained by power companies because of the many electric clocks and timers which depend on the power line frequency for timekeeping. However, a problem arises upon the failure, or outage, of electric power on the power lines providing electric power to the load being metered by the energy meter. When a power outage occurs, there is a loss of the AC power line frequency provided to the electronic register. A power fail detector is included in the electronic register to detect power outages, and to connect a carryover battery in circuit with the microprocessor to power the electronic register in the power down mode. However, the power fail detector detects the failure of the register power supply to provide DC voltage to the microprocessor. Upon a power outage the unregulated voltage across the main filter capacitor of the register power supply begins to decay as the filter capacitor discharges. When the DC voltage on the capacitor reaches a predetermined power fail threshold, a power fail interrupt signal is generated and sent to the microprocessor of the electronic register. The operation of the microprocessor is then "interrupted" and the computer enters the power down mode of operation which provides substitute timing means and backup battery power.
However, the filter capacitor of the register power supply is a large capacitor in order that the microprocessor will not respond to extremely short power outages. If the electronic register is using a minimum amount of power and the power line voltage is at a high level just before power is suddenly lost, the large filter capacitor is charged to a high level which can cause a significant delay before the capacitor discharges sufficiently to initiate the detection of a power fail interrupt. This can result in time being lost in the timekeeping function of the electronic register since timing pulses are not being supplied from the power lines during this period, and the substitute or backup timing has not yet been initiated. Such loss of timekeeping generates inaccuracies in the operation of the electronic register.